Principles of Computer Programming

• Computer hardware changes frequently - from room-filling machines with punch cards and tapes to modern laptops and tablets
• Capabilities of computers have changed rapidly (storage, speed, graphics, etc.)
• Languages used to program computers have also changed over time
  • Older languages: Fortran, C, C++
  • Newer “compiled” languages: C#, Java, R
  • Newer “interpreted” languages: Python, JavaScript
• This class is about “principles” of computer programming
  • Common principles behind all languages won’t change, even though hardware and languages do
  • How to organize and structure data
  • How to express logical conditions and relations
  • How to solve problems with programs

Programming Language Concepts

• Machine language
  • Computers are made of electronic circuits
  • Basic instructions are encoded by setting wires to “on” or “off”
    • Read data, write data, add, subtract, etc.
  • Binary digits represent on/off state of wires in a circuit
  • Machine language: which sequence of binary digits (circuit state) represents which computer instruction
    • Example instruction: 0010110010101101
  • Most CPUs use one of two languages: x86 or ARM
• Assembly language
  • Easier way for humans to write machine-language instructions
  • Use a sequence of letters/symbols to represent an instruction, instead of 1s and 0s.
    • Example x86 instruction: movq %rdx, %rbx
  • Assembler: Translates assembly language code to machine instructions
    • One assembly instruction = one machine-language instruction
    • x86 assembly produces x86 machine code
  • Computers can only execute the machine code
• High-level language
  • More human-readable than assembly language
  • Each statement does not need to correspond to a machine instruction
  • Statements represent more “high-level” concepts, such as storing a value in a variable, not “machine-level” concepts like “read these bits from this address”
  • Most languages we program in are high-level (C, C#, Python…)
  • Compiler: Translates high-level language to machine code
    • Small programs in high-level language might produce lots of machine code
    • Compiler is specific to both the source language and the target machine code
  • Compile then execute, since computers can only execute machine code
• Compiled vs. Interpreted languages
  • Not all high-level languages use a compiler - some use an interpreter
  • Interpreter: Lets a computer “execute” high-level code by translating one statement at a time to machine code
  • Advantage: Less waiting time before you can run the program (no separate “compile” step)
  • Disadvantage: Program runs slower, since you wait for each high-level statement to be translated before the program can continue
• Managed high-level languages (like C#)
  • Combine features of compiled and interpreted languages
  • Compiler translates high-level statements to intermediate language instructions, not machine code
    • Intermediate language: Looks like assembly language, but not specific to any CPU
  • Runtime executes compiled program by interpreting the intermediate language instructions - translates one at a time to machine code
  • Advantages of managed languages:
    • In a “non-managed” language, a compiled program only works on one OS + CPU combination (platform) because it is machine code
    • Managed-language programs can be reused on a different platform without recompiling - intermediate language is not machine code and not CPU-specific
    • Still need to write an intermediate language interpreter for each platform (so it produces the right machine code), but in a non-managed language you must write a compiler for each platform
    • Intermediate-language interpreter is much faster than a high-level language interpreter, so programs run faster than an “interpreted language” like Python
  • This still runs slower than a non-managed language (due to the interpreter), so performance-minded programmers use non-managed compiled languages (e.g. for video games)

Software Concepts

• Flow of execution in a program
  • Program receives input from some source, e.g. keyboard, mouse, data in files
  • Program uses input to make decisions
  • Program produces output for the outside world to see, e.g. by displaying images on screen, writing text to console, or saving data in files
• Program interfaces
  • GUI or Graphical User Interface: Input is from clicking mouse in visual elements on screen (buttons, menus, etc.), output is by drawing onto the screen
  • CLI or Command Line Interface: Input is from text typed into “command prompt” or “terminal window,” output is text printed at same terminal window
  • This class will use CLI because it’s simple, portable, easy to work with – no need to learn how to draw images, just read and write text

Programming Concepts

• Programming workflow (see flowchart)
  • Writing down specifications
  • Creating the source code
  • Running the compiler
  • Reading the compiler’s output, warning and error messages
  • Fixing compile errors, if necessary
  • Running and testing the program
  • Debugging the program, if necessary
• Intepreted language workflow (see flowchart)
  • Writing down specifications
  • Creating the source code
  • Running the program in the interpreter
  • Reading the interpreter’s output, determining if there is a syntax (language) error or the program finished executing
  • Editing the program to fix syntax errors
  • Testing the program (once it can run with no errors)
  • Debugging the program, if necessary
  • Advantages: Fewer steps between writing and executing, can be a faster cycle
  • Disadvantages: All errors happen when you run the program, no distinction between syntax errors (compile errors) and logic errors (bugs in running program)

Programming workflow

• Integrated Development Environment (IDE)
  • Combines a text editor, compiler, file browser, debugger, and other tools
  • Helps you organize a programming project
  • Helps you write, compile, and test code in one place
  • Visual Studio terms:
    • Solution: An entire software project, including source code, metadata, input data files, etc.
    • “Build solution”: Compile all of your code
    • “Start without debugging”: Run the compiled code
    • Solution location: The folder (on your computer’s file system) that contains the solution, meaning all your code and the information needed to compile and run it

Introduction to the C# Language

• C# is a managed language (as introduced in previous lecture)
  • Write in a high-level language, compile to intermediate language, run intermediate language in interpreter
  • Intermediate language is called CIL (Common Intermediate Language)
  • Interpreter is called .NET Runtime
  • Standard library is called .NET Framework, comes with the compiler and runtime
• It is widespread and popular
  • 7th most used language on StackOverflow, 5th-most if you discount JavaScript and HTML (which are used for websites, not programs)
  • .NET is the 2nd most used library/framework

The Object-Oriented Paradigm

• C# is called an “object-oriented” language
  • Programming languages have different paradigms: philosophies for organizing code, expressing ideas
  • Object-oriented is one such paradigm, C# uses it
  • Meaning of object-oriented: Program mostly consists of objects, which are reusable modules of code
  • Each object contains some data (attributes) and some functions related to that data (methods)
• Object-oriented terms
  • Class: A blueprint or template for an object. Code that defines what kind of data the object will contain and what operations (functions) you will be able to do with that data
  • Object: A single instance of a class, containing running code with specific values for the data. Each object is a separate “copy” based on the template given by the class.
  • Method: A function that modifies an object. This is code that is defined (written) in the class, but when it runs, it only runs on/for a specific object and modifies that object.
  • Attribute: A piece of data stored in an object
• Example objects:
  • “Car” object, represents a car
    • Attributes: Color, wheel size, engine status (on/off/idle), gear position
    • Methods: Press gas or brake pedal, turn key on/off, shift transmission
  • “Audio” object, represents a song being played in a music player
    • Attributes: Sound wave data, current playback position, target speaker device
    • Methods: Play, pause, stop, fast-forward, rewind

Hello World

/* I'm a multi-line comment,
 * I can span over multiple lines!
 */
using System;

class Program 
{
    static void Main() 
    {
        Console.WriteLine("Hello, world!"); // I'm an in-line comment.
    }
}

Features of this program:

• A multi-line comment: everything between the /* and /* is considered a comment, i.e. text for humans to read. It will be ignored by the C# compiler and has no effect on the program.
• A using statement: This imports code definitions from the System namespace, which is part of the .NET Framework (the standard library).
  • In C#, code is organized into namespaces, which group related classes together
  • If you want to use code from a different namespace, you need a using statement to “import” that namespace
  • All the standard library code is in different namespaces from the code you will be writing, so you’ll need using statements to access it
• A class declaration
  • Syntax: class [name of class], then { to begin the body of the class, then } to end the body of the class
  • All code between opening { and closing } is part of the class named by the class [name] statement
• A method declaration
  • The name of the method is Main, and is followed by empty parentheses (we’ll get to those later, but they’re required)
  • Just like with the class declaration, after the name, { begins the body of the method, } ends it
• A statement inside the body of the method
  • This is the part of the program that actually “does something”: It prints a line of text to the console
  • A statement must end in a semicolon (the class header and method header aren’t statements)
  • This statement contains a class name (Console), followed by a method name (WriteLine). It calls the WriteLine method in the Console class.
  • The argument to the WriteLine method is the text “Hello, world!”, which is in parentheses after the name of the method. This is the text that gets printed in the console: The WriteLine method (which is in the standard library) takes an argument and prints it to the console.
  • Note that the argument to WriteLine is inside double-quotes. This means it is a string, i.e. textual data, not a piece of C# code. The quotes are required in order to distinguish between text and code.
• An in-line comment: All the text from the // to the end of the line is considered a comment, and is ignored by the C# compiler.

Rules of C# Syntax

• Each statement must end in a semicolon (;),
  • Class and method declarations are not statements
  • A method contains some statements, but it is not a statement
• All words are case-sensitive
  • A class named Program is not the same as one named program
  • A method named writeline is not the same as one named WriteLine
• Braces and parentheses must always be matched
  • Once you start a class or method definition with {, you must end it with }
• Whitespace – spaces, tabs, and newlines – has almost no meaning
  • There must be at least 1 space between words
  • Spaces are counted exactly if they are inside string data, e.g. "Hello world!"
  • Otherwise, entire program could be written on one line; it would have the same meaning
  • Spaces and new lines are just to help humans read the code
• All C# applications must have a Main method
  • Name must match exactly, otherwise .NET runtime will get confused
  • This is the first code to run when the application starts – any other code (in methods) will only run when its method is called

Conventions of C# Programs

• Conventions: Not enforced by the compiler/language, but expected by humans
  • Program will still work if you break them, but other programmers will be confused
• Indentation
  • After a class or method declaration (header), put the opening { on a new line underneath it
  • Then indent the next line by 4 spaces, and all other lines “inside” the class or method body
  • De-indent by 4 spaces at end of method body, so ending } aligns vertically with opening {
  • Method definition inside class definition: Indent body of method by another 4 spaces
  • In general, any code between { and } should be indented by 4 spaces relative to the { and }
• Code files
  • C# code is stored in files that end with the extension “.cs”
  • Each “.cs” file contains exactly one class
  • The name of the file is the same as the name of the class (Program.cs contains class Program)

Reserved Words and Identifiers

• Reserved words: Keywords in the C# language
  • Note they have a distinct color in the code sample and in Visual Studio
  • Built-in commands/features of the language
  • Can only be used for one specific purpose; meaning cannot be changed
  • Examples:
    • using
    • class
    • public
    • private
    • namespace
    • this
    • if
    • else
    • for
    • while
    • do
    • return
• Identifiers: Human-chosen names
  • Names for classes (Rectangle, ClassRoom, etc.), variables (age, name, etc.), methods (ComputeArea, GetLength, etc), namespaces, etc.
  • Some have already been chosen for the standard library (e.g. Console, WriteLine), but they are still identifiers, not keywords
  • Rules for identifiers:
    • Must not be a reserved word
    • Must contain only letters (a → Z), numbers (0 → 9), and underscore (_)– no spaces
    • Must not begin with a number
    • Are case sensitive
  • Conventions for identifiers
    • Should be descriptive, e.g. “AudioFile” or “userInput” not “a” or “x”
    • Should be easy for humans to read and type
    • If name is multiple words, use CamelCase (or its variation Pascal case) to distinguish words
    • Class and method names should start with capitals, e.g. “class AudioFile”
    • Variable names should start with lowercase letters, then capitalize subsequent words, e.g. “myFavoriteNumber”

Write and WriteLine

• The WriteLine method

  • We saw this in the “Hello World” program: Console.WriteLine("Hello World!"); results in “Hello World!” being displayed in the terminal

  • In general, Console.WriteLine("text"); will display the text in the terminal, then start a new line

  • This means a second Console.WriteLine will display its text on the next line of the terminal. For example, this program:

    using System;
    
    class Welcome
    {
        static void Main()
        {
            Console.WriteLine("Hello");
            Console.WriteLine("World!");
        }
    }

    will display the following output in the terminal:

    Hello
    World!

• Methods with multiple statements
  • Note that our two-line example has a Main method with multiple statements
  • In C#, each statement must end in a semicolon
  • Class and method declarations are not statements
  • Each line of code in your .cs file is not necessarily a statement
  • A single invocation/call of the WriteLine method is a statement
• The Write method

  • Console.WriteLine("text") prints the text, then starts a new line in the terminal – it effectively “hits enter” after printing the text

  • Console.Write("text") just prints the text, without starting a new line. It’s like typing the text without hitting “enter” afterwards.

  • Even though two Console.Write calls are two statements, and appear on two lines, they will result in the text being printed on just one line. For example, this program:

    using System;
    
    class Welcome
    {
        static void Main()
        {
            Console.Write("Hello");
            Console.Write("World!");
        }
    }

    will display the following output in the terminal:

    HelloWorld!

  • Note that there is no space between “Hello” and “World!” because we didn’t type one in the argument to Console.Write

• Combining Write and WriteLine

  • We can use both WriteLine and Write in the same program

  • After a call to Write, the “cursor” is on the same line after the printed text; after a call to WriteLine the “cursor” is at the beginning of the next line

  • This program:

    using System;
    
    class Welcome
    {
        static void Main()
        {
            Console.Write("Hello ");
            Console.WriteLine("World!");
            Console.Write("Welcome to ");
            Console.WriteLine("CSCI 1301!");
        }
    }

    will display the following output in the terminal:

    Hello world!
    Welcome to CSCI 1301!

Escape Sequences

• Explicitly writing a new line

  • So far we’ve used WriteLine when we want to create a new line in the output

  • The escape sequence \n can also be used to create a new line – it represents the “newline character,” which is what gets printed when you type “enter”

  • This program will produce the same output as our two-line “Hello World” example, with each word on its own line:

    using System;
    
    class Welcome
    {
        static void Main()
        {
            Console.Write("Hello\nWorld!\n");
        }
    }

• Escape sequences in detail
  • An escape sequence uses “normal” letters to represent “special”, hard-to-type characters
  • \n represents the newline character, i.e. the result of pressing “enter”
  • \t represents the tab character, which is a single extra-wide space (you usually get it by pressing the “tab” key)
  • \" represents a double-quote character that will get printed on the screen, rather than ending the text string in the C# code.

    • Without this, you couldn’t write a sentence with quotation marks in a Console.WriteLine, because the C# compiler wouls assume the quotation marks meant the string was ending

    • This program won’t compile because in quotes is not valid C# code, and the compiler thinks it is not part of the string:

      class Welcome
      {
          static void Main()
          {
              Console.WriteLine("This is "in quotes"");
          }
      }

    • This program will display the sentence including the quotation marks:

      using System;
      
      class Welcome
      {
          static void Main()
          {
              Console.WriteLine("This is \"in quotes\"");
          }
      }

  • Note that all escape sequences begin with a backslash character (\)
  • General format is \[key letter] – the letter after the backslash is like a “keyword” indicating which special character to display
  • If you want to put an actual backslash in your string, you need the escape sequence \\, which prints a single backslash
    • This will result in a compile error because \U is not a valid escape sequence: Console.WriteLine("Go to C:\Users\Edward");
    • This will display the path correctly: Console.WriteLine("Go to C:\\Users\\Edward"); # Datatypes

Datatypes Nomenclature

• Value types
  • Numeric
    • Signed integer (sbyte, short, int, long)
    • Unsigned integer (byte, ushort, uint, ulong)
    • Real number (float, double, decimal)
  • Logical (bool)
  • Character (char)
• Reference types
  • String (string)
  • Object (object)

(In italics, the one we will mainly be using.)

Integers are “whole” numbers (ℤ = {…, -1, 0, 1, 2, 3, …}), floating point numbers are real numbers (ℝ), strings are “text messages”, …

Please refer to the “Datatypes in C#” cheatsheet for more information about datatypes.

String and Int Variables

Literals are fixed values (“Hi Mom”, 40, 1.2404, …) in the source code.

using System;

class MyFirstVariables
{
    static void Main()
    {
        // Declaration
        int myAge;
        string myName;

        // Assignment
        myAge = 40;
        myName = "Clément";
        
        // Displaying
        Console.WriteLine($"I am {myAge} old and my name is {myName}.");
    }
}

Variable Initialization

You can declare and assign a variable in one statement using what is called an “initialization statement”.

string myMessage = "Hey Mom";
int myValue = 12;

Remarks

• The value can change (hence the name!) if you re-assign it. The previously stored value is simply wiped out, and lost.

• You can store one variable’s value into another, but that value in the other variable won’t change when the original variable’s value changes:

  int a = 12;
  int b = a; // b's value is 12
  a = 0; // a's value changed to 0, but b's value is still 12.

• We can perform basic math operations with numeric datatypes: + (sum), * (multiplication), - (substraction) but also the modulo operation (%), which corresponds to the remainder. More details will be given in lab #3, in homework #2, and during lecture #4.

• There is a difference between

  int sum = num * 2; // The value of sum is num's value times two
  Console.WriteLine($"{sum}"); // The value of sum is displayed.

  and

  Console.WriteLine($"{num * 2}"); // The value of num times two is displayed, but the value of num is still the same.

• You can combine multiple declarations, initializations, and even mix both in one statement:

  int a=0, b, c;  // a, b, c are declared as three int variables, and a's value is set to 0.

Escape Sequences

The difference between Write and WriteLine.

Reading a String From the User

We can ask the user to give their name to the program while it is running as RunTime input. We can use a statement like:

string yourFirstName = Console.ReadLine();

to store the value entered by the user from the output window in the string variable labeled yourFirstName.

Reading an Int From the User

string test = "32";
int testConversion = int.Parse(test);

Console.WriteLine("Please enter the year.");
string answer = Console.ReadLine();
int currentYear = int.Parse(answer);
Console.WriteLine($"Next year we will be in {currentYear + 1}.");

Console.WriteLine("Please enter the year.");
int currentYear = int.Parse(Console.ReadLine());
Console.WriteLine($"Next year we will be in {currentYear + 1}.");

Console.WriteLine("Please enter the year.");
Console.WriteLine($"Next year we will be in {int.Parse(Console.ReadLine()) + 1}.");

Format Specifiers

We can use interpolation to display more nicely numerical values. There are four important format specifiers in C#.

        Console.WriteLine(
              "\n" + $"{1234.567:N}" // 1,234.57
            + "\n" + $"{1234.5:N}"   // 1,234.50
            + "\n" + $"{1234.567:E}" // 1.234567E+003
            + "\n" + $"{1234.567:C}" // $1,234.57
            + "\n" + $"{1234.5:C}"   // $1,234.50
            + "\n" + $"{.5:P}"       // 50.00%
    );

Introduction

Let us introduce a couple of key notions for object-oriented programming languages:

• We will be using classes and objects: A class is the specification (you can think of a cookie cutter, a blueprint, a model), and an object is the “actual thing” (the actual cookie my kid ate, my house, your car, …).
• To create an object from a class will need to instantiate it.
• An objet will be and do, that is, have data and operations. The class has attributes and methods (or procedures). When we instantiate, the object has instance variables.
• An object hides its structure: to have access to the value of the instance variables, you have to use properties (methods to access those attributes). Classes encapsulate the attributes and methods of the object and hides them (and the implementation details) from the other objects.
• Inheritance: a class can inherit from another class, i.e., extend it with new attributes or methods. A class can be extended in different ways, and those extensions can also be extended!

Code Sample

class Rectangle
{

/*
   A rectangle
    - has a lenght, a width, (attributes)
    - can be given a length, a width, can return its length, its width, and its area (methods).
    */

// Let us start with the attributes.

    private int length;
    private int width;
    
    // The key words "public" and "private" are "access modifiers".

// Now, for the methods.
// Every method will be of the form:
// public <return type> <Name>(<type of parameter> <name of parameter>){ <collection of statements>}

    public void SetLength(int lengthParameter)
    // Parameters are "local variables".
    {
        length = lengthParameter;
    }

    // This method will simply "take" the argument given, and store it as the length of the object.
    // Of course, a method could perform more advanced operations, like test the value, change it, compare it against other values, etc.

    public int GetLength()
    {
        return length;
    }

   public void Setwidth(int widthParameter)
    {
        width = widthParameter;
    }
    public int GetWidth()
    {
        return width;
    }

    public int ComputeArea()
    {
        return length * width;

    }
}

We will use this class in a separate file which contains a Main method that will create a rectangle object and manipulate it.

using System;

    class Program
    {
        static void Main(string[] args)
        {

        Rectangle myRectangle = new Rectangle(); // Instantiation
        myRectangle.SetLength(12); // Calling the classes' object.methodname(argument)
        myRectangle.Setwidth(3);

        Console.WriteLine($"You program's length is {myRectangle.GetLength()}" +
            $", its width is {myRectangle.GetWidth()}" + 
            $", so its area is {myRectangle.ComputeArea()}.");
        }
    }

Sometimes, “Parameters” are called “formal parameters” and “arguments” are called “actual parameters”. Stated differently, a method has parameters and takes arguments.

UML - Specification

Implementation

using System;

class ClassRoom
{
    private string name;
    private int number;

    public void SetName(string nameParameter)
    {
        name = nameParameter;
    }
    public string GetName()
    {
        return name;
    }

    public void SetNumber(int numberParameter)
    {
        number = numberParameter;
    }
    public int GetNumber()
    {
        return number;
    }
}

Default Values

What if we display the values of the instance variables before setting them?

ClassRoom english = new ClassRoom(); 
Console.WriteLine(english.GetName()); // Nothing!
Console.WriteLine(english.GetNumber()); // 0

Indeed, instance variables are different from “usual” variables in that sense that they receive a “default” value when created. This value depends of the variable datatype:

• Note how different it is from the variables we have been using so far, that could not be for instance displayed if their value had not been set.
• We can set a different default value, using, in the class declaration,

private string name = "Unknown";
private int number = -1;

Constructors

public ClassRoom(string nameParameter, int numberParameter)
{
    name = nameParameter;
    number = numberParameter;
}

ClassRoom math = new ClassRoom("Bertrand", 5);

public ClassRoom() { }

public ClassRoom() {
    name = "Unknown";
    int = -1;
}

First Example

Console.WriteLine("Enter your age");
int age = int.Parse(Console.ReadLine());
if (age >= 18)
{
    Console.WriteLine("You can vote!");
}

The idea is that the statement Console.WriteLine("You can vote!"); is exectued only if the condition (age >= 18) evaluates to true. Otherwise, that statement is simply “skipped”.

Syntax

if (<condition>)
{
    <statement block>
}

Please observe the following.

• <Condition> is something that evaluates to a bool. For instance, having a number like in if(3) would not compile.
• Note the absence of semicolon after if (<condition>).
• The curly braces can be removed if the statement block is just one statement.
• The following statements (that is, after the } that terminates the body of the if statement) are executed in any case.

Syntax

if (<condition>)
{
    <statement block 1>
}
else
{
    <statement block 2>
}

With if-else statements, the idea is that the statement block 1 is exectued only if the condition evaluates to true, and that the statement block 2 is exectued only if the condition evaluates to false. Note that since a condition is always either true or false, we know that at least one of the block will be executed, and since a condition cannot be true and false at the same time, at most one block will be executed: hence, exactly one block will be executed.

Example

We can make an example with really different conditions, not overlaping:

if (age > 12)
    x = 0;
else if (charVar == 'c')
    x = 1;
else if (boolFlag)
    x = 2;
else 
    x = 3;

Giving various values to age, charVar and boolFlag, we will see which value would x get in each case.

Example

We can make an example with really different conditions, not overlaping:

if (age > 12)
    x = 0;
else if (charVar == 'c')
    x = 1;
else if (boolFlag)
    x = 2;
else 
    x = 3;

Try to give various values to age, charVar and boolFlag, and see which value would x get in each case.

Formal Syntax

while (<condition>)
{
    <statement block>
}

Example

int number = 0;
while (number <=5)
{
    C.WL("Hi Mom!);
    C.WL(number);
    number++;
}

Notes:

• If <condition> is false: 0 execution of the body.
• If <condition> is always true: program loop for ever!
• The conditions under which <condition> changes should be given a chance to change in the body of the loop!

Failling to update the variable occuring in the condition

int number = 0;
while (number <=5)
{
    C.WL("Hi Mom!);
    C.WL(number);
}

Number isn’t changed!

Updating the “wrong” value

int number1, number = 0;
while (number <=5)
{
    C.WL("Hi Mom!);
    C.WL(number);
    number1++;
}

Having an empty body

int number = 0; 
while (number <=5); // Note the semi-colon here!
{
    C.WL("Hi Mom!");
    C.WL(number);
    number++;
}

Having an empty body (variation)

int number = 0;
while (number <=5)
    C.WL("Hi Mom!);
    C.WL(number);
    number++;

Going in the wrong direction

int number = 0;
while (number >=5)
{
    C.WL("Hi Mom!);
    C.WL(number);
    number++;
}

The variable number should be decremented, not incremented.

Motivation

Arrays are collection, or grouping, of values held in a single place. They can store multiple values of the same datatype, and are useful, for instance,

• When we want to store a collection of related values,
• When we don’t know in advance how many variables we need.

Declaration and Initialization of Arrays

Declaration and assignment

int[] myArray;
myArray = new int[3]; // 3 is the size declarator
// We can now store 3 ints in this array,
// at index 0, 1 and 2

myArray[0] = 10; // 0 is the subscript, or index
myArray[1] = 20;
myArray[2] = 30;

// the following would give an error:
//myArray[3] = 40;
// Unhandled Exception: System.IndexOutOfRangeException: Index was outside the bounds of the array at Program.Main()
// "Array bound checking": happen at runtime.

As usual, we can combine declaration and assignment on one line:

int[] myArray = new int[3];

We can even initialize and give values on one line:

int[] myArray = new int[3] { 10, 20, 30 };

And that statement can be rewritten as any of the following:

int[] myArray = new int[] { 10, 20, 30 };
int[] myArray = new[] { 10, 20, 30 };
int[] myArray = { 10, 20, 30 };

But, we should be carefull, the following would cause an error:

int[] myArray = new int[5];
myArray = { 1, 2 ,3, 4, 5}; // ERROR

If we use the shorter notation, we have to give the values at initialization, we cannot re-use this notation once the array was created.

Other datatype, and even objects, can be stored in arrays:

string[] myArray = { "Bob", "Mom", "Train", "Console" };
Rectangle[] arrayOfRectangle = new Rectangle[5];

Custom Size and Values

Console.WriteLine("What is size of the array that you want?");
int size = int.Parse(Console.ReadLine());
int[] customArray = new int[size];

How can we fill it with values, since we do not know its size? Using iteration!

int counter = 0;
while (counter < size)
{
    Console.WriteLine($"Enter the {counter + 1}th value");
    customArray[counter] = int.Parse(Console.ReadLine());
    counter++;
}

We can use length, a property of our array. That is, the integer value myArray.Length is the length (= size) of the array, we can access it directly.

To display an array, we need to iterate as well (this time using the Length property):

int counter2 = 0;
while (counter2 < customArray.Length)
{
    Console.WriteLine($"{counter2}: {customArray[counter2]}.");
    counter2++;
}

Changing the Size

Array is actually a class, and it comes with methods!

Array.Resize(ref myArray, 4);
myArray[3] = 40;
Array.Resize(ref myArray, 2);

Resize shrinks (and content is lost) and extends (and store the default value, i.e., 0 for int, etc.)!

for Loops

int i = 0;
while (i <= 5)
{
    Console.Write(i + " ");
    i++;
}

int j = 0;
do
{
    Console.Write(j + " ");
    j++;
} while (j <= 5);

int k = 0;
for (k = 0; k <= 5; k++)
{
    Console.Write(k + "");
}

for (int l = 0; l <= 5; l++)
{
    Console.Write(l + "");
}

Structure : initialization / condition / update

Ways Things Can Go Wrong

Don’t:

• Increment the counter in the body of the for loop!
• Assume that a variable declared in the header of a for loop will be accessible in the rest of the code. / Use for if you want to use the counter for anything else.
• Declare the variable twice.

For loops With Arrays

for loops actually go very well with arrays:

for (int i = 0; i < size; i++)
{
    Console.WriteLine($"Enter the {i + 1}th value");
    customArray[i] = int.Parse(Console.ReadLine());
}

Remember that we can use the Length property of our array. The previous code could become (only the first line changed):

for (int i = 0; i < customArray.Length; i++)
{
    Console.WriteLine($"Enter the {i + 1}th value");
    customArray[i] = int.Parse(Console.ReadLine());
}

Nested Loops

Of course, exactly as we could nest if statements, we can nest looping structures!

for (int o = 0; o < 11; o++)
{
    for (int p = 0; p < 11; p++)
        Console.Write($"{o} × {p} = {o * p}  \t ");
    Console.Write();
}

Mixing Control Flows

And we can use if statements in the body of for loops:

for (int m = 0; m < 10; m++)
{
    if (m % 2 == 0) Console.WriteLine("This is my turn.");
    else Console.WriteLine("This is your turn.");
}

Iterations

There is another, close, structrure that allows to iterate over the elements of an array, but can only access them, not change their values (they are “read only”).

for (int i = 0; i < myArray.Length; i++)
    Console.Write(myArray[i] + " ");

foreach (int i in myArray) // "Read only"
    Console.Write(i + " ");

Diffference is w.r.t. to modifying the array “read Vs write”. Having i = 2 in the foreach would cause an error!

That last structure is given for the sake of completeness, but it’s ok if you’d rather not use it.

Static Class Members

Class member = methods and fields (attributes)

Motivation: the methods we are using the most (WriteLine, ConsoleRead) are static, but all the methods we are writting are not (they are “non-static”, or “instance”).

A static class member is associated with the class instead of with the object.